An ultrasonic sensor is mounted on an automotive vehicle, for example. The sensor detects a distance between the sensor, i.e., the vehicle and an obstruction when a driver parks the vehicle or when the driver turns the vehicle. The ultrasonic sensor is disclosed in, for example, JP-A-2001-16694. The sensor for detecting the obstruction includes a transmission device and a reception device, which transmits an ultrasonic wave and receives the ultrasonic wave. The sensor may include a transmitting/receiving device. When the transmission device transmits the ultrasonic wave, the ultrasonic wave hits the obstruction. The obstruction reflects the ultrasonic wave; and then, the reflected ultrasonic wave is received by the reception device. On the basis of the received ultrasonic wave by the reception device, an acoustic pressure of the ultrasonic wave, a time lag and/or a phase difference are detected so that a direction to the obstruction and a distance between the obstruction and the vehicle are calculated. Further, a concavity and a convexity of the obstruction can be detected.
The reception device of the ultrasonic wave is for example, an ultrasonic element having a vibrator formed of a piezoelectric thin film disposed on a membrane as a thin portion of a substrate. The ultrasonic element with a membrane structure is disclosed in, for example, JP-A-2003-284182. This element is formed by a micro machining method so that the element is called a MEMS (i.e., micro electro mechanical system) type ultrasonic element. JP-A-2003-284182 also discloses an ultrasonic array sensor including the MEMS type ultrasonic elements.
The MEMS type ultrasonic element 90R is shown in FIG. 13A. In the element 90R, a PZT ceramics thin film layer 2 as a ferroelectric substance is sandwiched by a pair of electrodes 3a, 3b. The element 90R further includes a piezoelectric sensor having a predetermined resonance frequency for detecting the ultrasonic wave. When the element 90R operates, a predetermined bias voltage is applied between two electrodes 3a, 3b so that the resonant frequency of the element 90R is changed, i.e., controlled.
FIG. 13B explains a positioning measurement method by using the ultrasonic wave, which is disclosed in JP-A-2003-284182. An ultrasonic sensor 900 includes an ultrasonic wave source 40 as a transmission device of the ultrasonic wave and an ultrasonic array device A90R as a reception device of the ultrasonic wave. The ultrasonic array device A90R includes multiple MEMS type ultrasonic elements 90R, which are arrayed. In the sensor 900, the source 40 is adjacent to the sensing device A90R, and transmits the ultrasonic wave. The ultrasonic wave hits an object 51, 52 as an obstacle; and then, the ultrasonic wave is reflected by the object 51, 52. Thus, the ultrasonic wave is returned to the sensor 900. The returned ultrasonic wave is received by each sensing element 90R in the sensing device A90R. On the basis of the received ultrasonic wave, the position of the object 51, 52 including an orientation angle to the object 51, 52 is determined. Specifically, on the basis of a transmission time of the ultrasonic wave in each incident direction of the sensing element 90R, the distance between the sensing element 90R and the object 51, 52 in the incident direction is calculated. Thus, distribution of the distance in different incident directions is determined. Accordingly, the distance between the object 51, 52 and the sensing element 90R in a depth direction of the object 51, 52 is determined. Here, the transmission time of the ultrasonic wave is a time from a transmission time when the ultrasonic wave is transmitted from the source 40 to a returning time when the ultrasonic wave is returned to the sensing element 90R.
Here, the source 40 and the sensing device A90R are separated each other. Therefore, a manufacturing cost of each of the source 40 and the sensing device A90R is necessitated. Further, when the source 40 and the sensing device A90R are mounted on a bumper of the vehicle, mounting accuracy of each of the source 40 and the sensing device A90R affects detection accuracy of the direction and the distance of the object. Furthermore, the mounting distance between the source 40 and the sensing device A90R may be increased.
Further, in general, when an ultrasonic sensing device is directly mounted on the bumper of the vehicle, the sensing device cannot detect the distance to the object accurately by a water drop or a dust attached on a surface of the sensing element. Furthermore, attenuation of the ultrasonic wave transmitting through air depends on temperature and humidity of the air. These temperature and humidity are changeable in accordance with the environment around the vehicle. Thus, the detection accuracy of the object may depend on temperature change and humidity change. Specifically, the environmental temperature around the vehicle can be detected by an external temperature sensor or the like. However, there is no appropriate external humidity sensor mounted on the outside of the vehicle. Thus, the environmental humidity around the vehicle cannot be detected.